1. Field of the Invention
The present invention relates to a vehicle having an electric drive which is supplied by an external electric supply system, and at least one battery, a method for operating such a vehicle, and a method for retrofitting a vehicle with an electric drive which is supplied by an external electric supply system.
2. Relevant Art
Urban local and regional passenger transportation vehicles such as trams or trolley buses generally do not travel on separate roadways but instead are operated together with the individual vehicular traffic, i.e., automobiles, on the same roadway, generally a road. These public transportation vehicles are usually electrically driven and supplied with electric power via contact lines, for example, overhead lines. When there is a power outage, due, for example, to a rupture of a power line or a defective current collector, etc., there is the considerable risk of the traffic being impeded since the public transportation vehicle can no longer move independently, which is very critical in urban situations and can result in considerable financial loss as a result of congestion of individual vehicular traffic.
The fact that such vehicles conventionally cannot be operated independently without an external power supply, can lead to other problems in certain situations, for example, starting up when the vehicle happens to be stationary with its current collectors under section isolators, in addition to when there is a power failure. It is also impossible to carry out voltageless shunting movements in the depot. In that situation, expensive auxiliary vehicles have always been necessary.
One known solution to this problem are so-called hybrid vehicles which are capable of being driven by an external electric supply, and have an additional power supply such as a diesel powered electric generator which can be used to supply the drive of the vehicle when necessary. These, however, are respectively fully functional power supply systems or a fully functional drive with which the vehicle can be moved at least over a certain time period without limiting the load or speed. Such a vehicle is known, for example, from WO 93/04887 A1 and WO 00/77918 A1. Since the power supply systems in these vehicles have to be at least approximately equivalent, they of course also have to be given equivalent dimensions, which increases the costs for the backup drive, and thus for the vehicle.
Vehicles of the type in question generally carry on-board battery systems used to supply the fundamental control functions of the vehicle, such as, for example, the general control system, the raising and lowering of the current collector, the vehicle lighting system, etc. It would be particularly advantageous if this battery system could also be used to provide emergency traction power when necessary. However, the voltage of the on-board battery system is very much lower than the external traction supply voltage, e.g., 24 Volts compared to 600 Volts. Up to now, a practical way of using the on-board battery system for emergency traction power has not been known.